SPST vs SPDT

Dear,

i received a submittal showing motorized valve with SPST instead of SPDT which is required. supplier is unable to comply.
what is the real advantage of having SPDT instead of SPST, from how i see it, SPDT provide flexibility to have NC and NO to the PLC. and maybe if one set of contact is damaged, we can use the other of contacts.

your feedback is highly valuable,

charbel

SPDT gives you an advantage in that it gives you the advantage of wire disconnection detection. I don't want to call it "fail safe" because it's not, but it's MORE fail safe than a NO contact. If your PLC is monitoring a channel for power, you know that a wire is connected. If the power drops out at the wrong time, you know a wire fell off (or something is wrong with the circuit). Usually, on a SPDT device, one contact or the other is used, but not usually both. The reason you would use both is to add a confirmation that the contact actually changed states and isn't just being bypassed by a simple jumper, but usually this is only really needed on a safety application, and in that case a safety device has more than just SPDT that make it rated for that application.

The BEST way to check for wire-fall-off and contact weld situations is a DPDT mechanically-linked set of contacts.

The answer in your situation, of course, is what are you using these contacts for? If they're being used for travel limits on this motorized valve, you will definitely want to use NC contacts to protect the motor from ramming it's mechanical stop if a wire falls off or gets disconnected from the travel limit. Now, SPST COULD mean just a single NC contact, but usually SPDT would be ideal. I would use the NO contact as a status input to the PLC, and wire the NC contact physically in line with each direction wiring of the coil so it is a hard-wired stop at the travel limits.

...from how i see it, SPDT provide flexibility to have NC and NO to the PLC.

Click to expand...

Yes, but normally only 1 PLC input will be allocated and available for each switch, so you could only use 1 part of a Single-Pole Double-Throw contact at a time.

I would not let the lack of the extra pole be the deciding factor. If the valve meets your specification in all other areas, then I might just make a revision in the specification, and reissue it to all bidders, so that they all have a fair chance to bid with only SPST limit switches.

SPDT gives only the possibility to have NO or NC not to change the contacts as for this you will need DP
best is DPDT
one set to limit switch and the otherset for plc input.